Gases

Since landfills are anaerobic biological reactors, they produce CH₄ and CO₂.
The first stage is aerobic and may last from a few days to several months, during which time aerobic organisms are active and affect the decomposition.
As the organisms use up the available oxygen, the landfill enters the second stage, when anaerobic decomposition begins but methane-forming organisms have not yet become productive.
During the second stage, the acid formers cause a build up of CO₂.
In methane production stage, during which the percentage of CH₄ progressively increases and the landfill interior temperature rises to about 55°C.
The final, steady-state condition occurs when the fractions of CO₂ and CH₄ are about equal and microbial activity has stabilized.
Unwanted gas migration may be prevented by installing escape vents in the landfill
Landfill gas can be burned to produce electric power. Alternatively, the gas can be cleaned of CO2 and other contaminants and used as pipeline gas.